Emergency power supply systems are known, which are based on a ram air turbine (RAT), which is mechanically coupled with a hydraulic pump for providing hydraulic power to actuators of a flight control system. For providing electric power, a hydraulic powered generator is coupled with the hydraulic system.
The concept of providing a fuel cell system instead of a generator for providing electrical power in an emergency case is known e.g. from EP 1 859 499 B1 and EP 2 222 562 B1.
A replacement of a ram air turbine with a fuel cell requires modifications of the electric system or the hydraulic system, as a fuel cell is only able to provide a direct current having a voltage clearly below a required voltage for generating an alternating current with a common voltage, such as 115 V at 400 Hz. Also, the fuel cell only delivers a direct current, which additionally depends on the electrical load applied to the fuel cell.
Furthermore, common electric hydraulic pumps also require a rather high voltage and, especially during start-up, a great current. Consequently, following a conservative design approach, a fuel cell would comprise a distinct size and weight and a variable DC output voltage would have to be accepted.